iHeartRadio Music Festival 2012 Day One: Britney Spears, No Doubt, Demi Lovato and More (PHOTOS)

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There was plenty of star power to go around on Friday as the annual iHeartRadio Music Festival kicked off its 2-day extravaganza with help from Britney Spears, No Doubt, Demi Lovato, host Ryan Seacrest and more.

Spears, 30, showed up for the festival, held at Las Vegas’ MGM Grand, in a skin tight LBD and rainbow colored tips.

Onstage, Gwen Stefani and company delivered several tunes in their opening set, including a special performance of “Just a Girl” with Pink.

What other big names showed up for the music festival?

Usher mingled with Korean sensation Psy backstage before both delivered separate performances onstage.

Ryan Lochte, Rihanna, Lil Wayne and Green Day were also on hand for the big show.

Taylor Swift and Aerosmith will headline the festival’s second day of music on Saturday.

Click through the gallery above for highlights from day one of the iHeartRadio Music Festival.